As operation speed of electronic devices become increasingly faster, an important issue is how to achieve higher integration (miniaturization) and higher efficiency in circuits handling microwaves (frequencies above the 1 GHz band). One typical example of a microwave oscillating source is the Gunn oscillator. The Gunn oscillator has the advantages of being able to operate at low voltages, and of having a high oscillation spectral purity (i.e., the percentage of frequency components outside the desired oscillation frequency is small). The Gunn oscillator, however, has the disadvantages of difficulty of size reduction for structural reasons and of poor oscillation efficiency (output power/input power=1% or less). For these reasons, it is now becoming more mainstream to use a method whereby an oscillation produced by a semiconductor element such as a transistor or a PIN diode is multiplied to provide a high frequency.
Further, in microwave circuits, the improvement of oscillation (or detection) efficiency of the oscillator (or detector) is not enough; a significant problem is now being posed by the loss through transmission lines due to impedance mismatch, which is caused by the decrease in the size of microwave transmission lines (consisting of, e.g., striplines or coaxial cables) associated with the increases in frequency (namely, wavelengths become shorter as the frequencies become higher).
Various proposals have so far been made in order to improve the efficiency of coupling with transmission lines, such as an oscillation element consisting of a Gunn oscillator and a stripline that are formed in a module (Patent Document 1), and an oscillator having a semiconductor element formed on a microwave transmission line (Non-patent Document 1: a flip-chip type Gunn diode that can be fabricated on a planar substrate). None of those proposals are capable of achieving significant improvement of oscillation efficiency over conventional technologies.
Thus, the major problem of a semiconductor-element microwave oscillator results from low oscillation efficiency and impedance mismatch between the oscillator and the transmission line. In the case of oscillation by a semiconductor oscillation element, there is also the problem of frequency purity in oscillation output.
It has recently been discovered that magnetization reversal can be caused by the current in a CPP-GMR (giant magnetoresistance) element. The CPP-GMR element herein refers to a current-perpendicular-to-plane giant magnetoresistance element, where a magnetic multilayer film having a magnetization free layer, an intermediate layer, and a magnetization fixed layer is formed perpendicular to the film plane in a columnar shape such that the current flows in a direction perpendicular to the film plane. Magnetoresistance is a phenomenon in which the direction of magnetization in the magnetization free layer changes upon application of an external magnetic field, whereby the resistance value of the element is changed. It has so far been believed that resistance in a magnetoresistance element can be changed only through the application of external magnetic field to change the direction of magnetization in the magnetization free layer. Therefore, it was a new discovery that the direction of magnetization in the magnetization free layer can be changed by current alone.
Such magnetization reversal by current is based on the resonance oscillation of the spins in the magnetization free layer. It has been reported that microwaves are produced upon excitation of resonance, and that the frequencies of the microwaves vary depending on external magnetic field. In Non-patent Document 2, the generation of microwave in a CPP-GMR element consisting of the three layers of Co, Cu, and Co is reported. The oscillation frequency of the microwaves obtained in the experiment in Non-patent Document 2 was on the order of 10 GHz to 25 GHz.
It has also been reported that current injection magnetization reversal requires a minute cross-sectional area (on the order of, e.g., 100 nm×200 nm or less for the Co, Cu, and Co three-layer film, for example) such that the magnetization of the magnetization free layer can become a single magnetic domain state. Such magnetization reversal is caused by the magnetization of the magnetization free layer producing a resonance oscillation based on the spin torque produced by the flow of current. It has been reported that, even in a current region in which no magnetization reversal occurs, oscillation of microwaves (on the order of approximately 10 GHz) is taking place in the magnetization free layer due to spin torque.
Since such oscillation is based on the collective motion of the electron spins in the magnetization free layer, it is expected that essentially the Q value (an index of the sharpness of resonance in a resonance circuit) will increase. Thus, it is expected that if such resonance oscillation can be utilized as a microwave oscillation source, higher efficiency can be achieved compared with existing microwave oscillation sources.
In the following, problems associated with the detection of microwaves are discussed. For the detection of microwaves, normally the quadratic detection characteristics of a semiconductor diode are utilized. To perform detection with high efficiency, it is necessary that there be no delay in the motion of electrons within the semiconductor. For this reason, a semiconductor or a PIN diode that has high mobility is employed. While it is possible to achieve high frequencies by decreasing electron channel length (i.e. by reducing the thickness of an element), the resultant structure would be subject to an increase injunction capacitance. If the element area is reduced in order to decrease the junction capacitance, this will then result in an increase in element resistance, thereby producing the problem of a decrease in sensitivity due to impedance mismatch with the transmission line.
Furthermore, since the quadratic characteristics of a semiconductor diode greatly depend on temperature, it is difficult to obtain stable sensitivity. Thus, even if the aforementioned problems in semiconductor diode detection in the microwave region (i.e. delay in electron motion, junction capacitance, and impedance mismatch) have all been resolved, there still remain the problem of detection efficiency characteristics being limited by temperature.    Patent document 1: JP Patent Publication (Kokai) No. 2000-353920 (P2000-353920A), entitled: Gunn diode oscillator    Non-patent document 1: Flip-chip type Gunn diode, Atsushi Nakagawa, Ken-ichi Watanabe, “Flip-Chip Gunn Diode,” Oyo Buturi, vol. 69, No. 2 (2000), p. 182.    Non-patent document 2: S. I. Kiselev, J. C. Sankey, I. N. Krivorotov, N. C. Emley, R. J. Schoelkopf, R. A. Buhrman & D. C. Ralph, “Microwave oscillations of a nanomagnet driven by a spin-polarized current,” Nature vol. 425, (2003) pp. 380.